Non-tempered steel for mechanical structure

ABSTRACT

The medium carbon, microalloyed forging steel for machine structural use has a small deformation upon fracture in a hot-rolled, hot-forged, or any other hot-worked state, has an inexpensive ferrite-pearlitic microstructure and consists of C: 0.3 to 0.6 wt %, Si: 0.1 to 2.0 wt %, Mn: 0.1 wt % or more and less than 0.4 wt %, P: 0.01 to 0.1 wt %, S: 0.01 to 0.2 wt %, V: more than 0.15 wt % and up to 0.4 wt %, and the balance: Fe and unavoidable impurities, in which the unavoidable impurities include less than 0.005 wt % N. The microalloyed forging steel for machine structural use may further contain Al: 0.005 to 0.05 wt %, one or both of Ti: 0.005 to 0.05 wt % and Nb: 0.05 to 0.2 wt %, and/or one or both of Cr: 0.1 to 0.5 wt % and Mo: 0.1 to 0.5 wt %.

TECHNICAL FIELD

The present invention relates to a microalloyed forging steel formachine structural use having a small deformation of a fracture surfacewhen fracture-split and is generally applicable to steel blanks formachine structural use and machine parts which require a smalldeformation upon tensile and impact fracture.

BACKGROUND ART

The steels for machine structures, which are used to form parts ofautomobiles and industrial machinery, are usually supplied in the formof a straight bar or a coiled wire and are hot- or cold-worked to adesired shape, followed by various heat treatments, machining, etc., toprovide a final part. When the processing from steel blanks to partsincludes fracture-separation by cold tension, it is usually necessary tocontrol the deformation upon fracture in order to ensure the requiredprecision in the subsequent working step or to prevent occurrence oftroubles in an automated working line.

Usual steel parts were conventionally formed by hot or cold forging,followed by quench-hardening and tempering to provide required strengthand toughness. These days, microalloyed steels for hot forging(hereinafter simply referred to as "microalloyed forging steel"), whichhave the required strength in an as-forged state, are increasingly used.Replacing the quench-hardened and tempered steel with the microalloyedforging steel is advantageous because omission of heat treatment lowersthe production cost and eliminates quenching distortion.

The forming method of microalloyed forging steel parts includesfracture-splitting by impact tension, working of required portions andthen recoupling the fracture surfaces and is practically used typicallyfor forming a connecting rod made, for example, of a steel having arelatively high carbon content such asFe-0.72%C-0.22%Si-0.49%Mn0.062%S-0.04%V as described in "Fundamentalsand Applications of Microalloying Forging Steels", (1996) 29 TMS.

The process of producing a connecting rod can be roughly summarized ashot-forging of a steel blank followed by air cooling, boring anddrilling of a cap and a rod, mechanical splitting of a large end,recoupling of the fracture surfaces, bolting of the cap and the rod, andfinish-machining.

This process is advantageous because relatively inexpensive steel blankscan be used and conventionally required high precision machining canalso be omitted to reduce costs. However, the above-recited steelcontains a large amount of carbon to enhance fracturability, andtherefore, has a problem of low yield strength and fatigue strength aswell as poor machinability.

Japanese Unexamined Patent Publication (Kokai) No. 8-291373 discloses asteel, for connecting rods, in which the carbon content is reduced fromthe above-recited steel while fracturability is ensured, and describesthat the disclosed microalloyed steel for hot forging is "easy tofracture-separate and the fractured surface has a small deformation andis easily recoupled".

Japanese Unexamined Patent Publication (Kokai) No. 9-3589 discloses alow toughness microalloyed forging steel for connecting rods anddescribes that an increased N amount, in particular, provides a brittlefracture surface upon fracture-splitting and "the object is to provide ahigh strength, low toughness microalloyed forging steel which exhibits aflat, brittle fracture surface when fracture-split at room temperature".

However, the steels disclosed in Japanese Unexamined Patent Publication(Kokai) No. 8-291373 or Japanese Unexamined Patent Publication (Kokai)No. 9-3589 failed to provide a commercially acceptable fracturability.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide an inexpensive, mediumcarbon microalloyed forging steel for machine structural use having asmall deformation when fractured in the state of as hot-worked by hotrolling, hot forging, etc. and being composed of a ferrite-pearliticmicrostructure.

To reduce the deformation of a steel upon fracture, it is most effectiveto reduce the ductility of the steel. Several measures are able toreduce the ductility by adjusting the chemical composition of steel. Oneis to increase the carbon content such as the 0.72% C steel described inthe above-recited (1996) 29 TMS. However, steels having aferrite-pearlitic microstructure have a lower yield ratio (yieldstrength/tensile strength) and a lower fatigue strength as the carboncontent is increased. Another is to use a large amount of P to embrittlecrystal grain boundaries but P also significantly reduces the ductilityat high temperatures and makes difficult the casting, rolling and hotworking of steel.

To provide an improved fracturability without the above problems, thepresent inventors conducted various studies and obtained the followingnovel findings.

1) Improvement of Fracturability

Mn acts as a solid solution strengthening element to strengthen a steelwhile causing no significant reduction in ductility due to thestrengthening and the medium carbon (0.25% or more C) steels for machinestructural use usually contain about 0.6% or more Mn. Based on thisfact, the present inventors studied the relationship between Mn andfracturability and found that there is a strong correlation between thefracturability and the Mn content, particularly when the Mn content isreduced to less than 0.4%, the steel ductility is lowered and thedeformation upon fracture is reduced. The reduced Mn contentadvantageously lowers the ductility while causing no significantreduction in the high temperature ductility, which is different from theaddition of a large amount of P.

Microalloyed forging steels generally contain V or Nb as a precipitationstrengthening element and, if these elements are bonded with N in steelto form nitrides, austenite grains are refined during heating forforging and the ferrite amount in the microstructure is also increasedto increase the ductility, so that the reduction in Mn content alonecannot provide the practically required low ductility (highfracturability). Therefore, it is of primary importance to suppressprecipitation of nitrides by reducing the N content. Some ofmicroalloyed forging steels designed for improved toughness contain0.01% or more N, and even otherwise, steels obtained by the usualsteelmaking process usually contain 0.005% or more N. JapaneseUnexamined Patent Publication (Kokai) No. 9-3589 recommends addition ofN in as large an amount as possible. However, the present inventorsconducted experiments using V added, 0.5% C microalloyed forging steelsand found that the deformation in terms of the reduction of the fracturesurface area is smaller for lower N contents such that a 0.004% N steelhas a deformation of 70 taking that of a 0.01% N steel as 100.

2) Improvement of Yield Strength and Fatigue Strength

To provide a ferrite-pearlitic steel with improved yield ratio (yieldstrength/tensile strength) and fatigue limit ratio, it is effective toreduce the carbon content and increase amounts of suitable alloyingelements. In V-strengthened microalloyed forging steels, simply reducingthe carbon content from 0.7% to 0.6% improves the yield ratio from 0.55to 0.65 and the fatigue limit ratio from 0.39 to 0.44. Thus, it isimportant to reduce the carbon content as long as the requiredfracturability is ensured. As is known in the art, improving the yieldratio and the fatigue limit ratio by the precipitation strengtheningeffect of V is also essential to make up for the strength reductionbecause of the reduced C and Mn contents.

Based on the above findings, the first, second, third and fourthinventions provide microalloyed forging steels for machine structuraluse as stated in (1), (2), (3) and (4) below.

(1) A microalloyed forging steel for machine structural use,characterized by consisting of:

C: 0.3 to 0.6 wt %,

Si: 0.1 to 2.0 wt %,

Mn: 0.1 wt % or more and less than 0.4 wt %,

P: 0.01 to 0.1 wt %,

S: 0.01 to 0.2 wt %,

V: more than 0.15 wt % and up to 0.4 wt %, and

the balance: Fe and unavoidable impurities, in which the unavoidableimpurities includes less than 0.005 wt % N and the steel has aferrite-pearlitic microstructure.

(2) A microalloyed forging steel for machine structural use as stated in(1), characterized by further containing:

Al: 0.005 to 0.05 wt %.

(3) A microalloyed forging steel for machine structural use as stated in(1) or (2), characterized by further containing one or both of:

Ti: 0.005 to 0.05 wt %, and

Nb: 0.05 to 0.2 wt %.

(4) A microalloyed forging steel for machine structural use as stated inany one of (1) to (3), characterized by further containing one or bothof:

Cr: 0.1 to 0.5 wt %, and

Mo: 0.1 to 0.5 wt %.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a fracture surface of a notched tensiletest piece broken by tension (cross-section 10×20 mm, notch root 1.0R,notch depth 2.0 mm), in which A is a length perpendicular to the notchand B and C are lengths parallel to the notch.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, the chemical composition isspecified for the following reasons.

C: 0.3 to 0.6%

0.3% or more C is necessary to provide a required strength of machinestructural parts and an improved fracturability by embrittlement of thesteel. However, an excessive C content lowers yield strength and fatiguestrength, and therefore, the upper limit is 0.6%.

Si: 0.1 to 2.0%

Si acts as a solid solution strengthening element, also lowers the steelductility and must be present in an amount of 0.1% or more to providesignificant reduction in ductility. However, an amount more than 2.0%lowers the high temperature ductility to cause cracking to occur duringrolling and forging and also promotes decarburization.

Mn: 0.1% to less than 0.4%

Mn is usually used as a solid solution strengthening element, and in thepresent invention, the Mn content is limited to less than 0.4% to lowerthe ductility. Mn also forms MnS to improve machinability. However, ifthe Mn content is less than 0.1%, S is brought into solid solution toembrittle crystal grain boundaries during heating and the hot ductilityis lowered to cause frequent occurrence of cracking during production ofsteel blanks and steel parts.

P: 0.01 to 0.1%

P is segregated at crystal grain boundaries cause embrittlement of thesteel, thereby improving the fracturability. To provide this effect, theP content must be 0.01% or more. However, an excessive P content lowersthe hot ductility and causes cracking to easily occur, and therefore,the P content must not be more than 0.1%.

S: 0.01 to 0.2%

S is used to improve the machinability. The S amount must be 0.01% ormore to improve the machinability and the upper limit is 0.2% tosuppress development of anisotropy of the mechanical properties.

V: more than 0.15% to 0.4%

V mainly improves the yield strength and the fatigue strength byprecipitation strengthening and also lowers the ductility. V must bepresent in an amount of more than 0.15% but a V amount of more than 0.4%only provides a small effect with respect to the required cost.

N: less than 0.005%

Reduction of the N content is very important to provide an improvedfracturability. N forms VN and NbN to refine the microstructure of steelblanks and hot-worked products and also increases the ferrite amount toenhance the ductility, and therefore, the N amount is preferably assmall as possible. To provide a practically required small deformationupon fracture, the N content must be less than 0.005%.

Al: 0.005 to 0.05%

Al acts as a deoxidizing agent. Usual forging steels are produced byusing Al deoxidation, which unavoidably causes dispersion of aluminaparticles in the steel to occasionally lower the machinability.Therefore, when a very good machinability is required, Al deoxidation isnot used (the first invention). The omission of Al deoxidation alsoadvantageously ensures absence of the precipitation of AlN, so that themicrostructure is coarsened to improve the fracturability.

However, when a target tensile strength is low, or when the machiningamount is small, the machinability raises no significant problems and0.0005% or more Al may be present, but an amount more than 0.05%provides no further effect (the second invention).

Ti: 0.005 to 0.05%

Ti is utilized as a precipitation strengthening element. If TiN isformed, the hot-forged microstructure is refined to enhance theductility. However, a required low ductility is obtained if the Ncontent is less than 0.005% and the steel has a sufficiently highhardness. To ensure precipitation strengthening, 0.005% or more Ti isnecessary and the upper limit is less than 0.05% to prevent lowering ofthe machinability because of formation of coarse oxides.

Nb: 0.05 to 0.2%

Similarly to V, Nb provides precipitation strengthening to improve theyield strength and the fatigue strength and to lower the ductility. Thepresence of Nb together with V further improves the above effect. The Nbcontent must be 0.05% or more to effect strengthening but an Nb amountof more than 0.2% only provides a small effect with respect to therequired cost.

Cr: 0.1 to 0.5%, Mo: 0.1 to 0.5%

Cr and Mo may be added in an amount of 0.1% or more, respectively, ifnecessary for adjustment of the strength, and the amount must not bemore than 0.5% to prevent the fracturability from lowering because ofrefinement of a pearlitic microstructure.

It would cause no problem if, to improve the machinability, one or moreof 0.01 to 0.4% Pb, 0.01 to 0.4% Bi, 0.01 to 0.04% Se, 0.002 to 0.005%Te and 0.0005 to 0.003% Ca are added in the present inventive steel.

Ferrite-pearlitic steels have a tensile strength and a hardness whichare basically determined by the carbon equivalent Ceq. expressed by aformula such as Ceq.(%)=C%+(1/7)Si%+(1/5)Mn%+(1/2)V% described inJapanese Examined Patent Publication (Kokoku) No. 60-45250. As can beseen from this formula, the present inventive steel is inexpensivebecause it is a medium carbon steel and a desired tensile strength canbe achieved by using small amounts of expensive elements other thancarbon. The production cost is also substantially reduced by using thepresent inventive steel to produce steel parts by hot forging withoutsubsequent heat treatment.

The present inventive steel is further characterized by having aferrite-pearlitic microstructure, which requires no special steelmakingprocess or forging method but is achieved by a usual commercialsteelmaking process including melting and casting and a usual hotrolling to a hot-rolled bar or a hot forging to form automobile parts,followed by free air cooling or fan-forced air cooling. It is a furtheradvantage of the present inventive steel that it has a medium carbon,low Mn composition containing V facilitating ferritic transformation,and therefore, supercooled phases such as bainite hardly form incontrast to the conventional microalloyed steel for hot forging.

EXAMPLES

Steels having chemical compositions summarized in Table 1 were producedby using a 150 kg vacuum melting furnace, reheated at 1473 K, hot-forgedto round bars having a diameter of 20 mm, and air-cooled to providesteel blanks. All of the samples had a ferrite-pearlitic microstructure.To measure the deformation upon fracture, notched tensile test pieces(cross-section: 10×20 mm, notch root radius: 1.0 R, notch depth: 2.0 mm)were machined from the steel blanks and were fractured by tension.Measurement of the fractured test pieces showed that all of the sampleshad substantially the same deformation on the fracture surface in thedirection perpendicular to the notch (the change in the length of edge Ashown in FIG. 1). The fracturability was evaluated ("deformation" inTable 1) in terms of the deformation on the fracture surface in thedirection parallel to the notch, specifically the sum of the changes inwidth of the fracture surface on the notch side and on the smooth side(the changes in the lengths of edges B and C shown in FIG. 1). Unnotchedtensile test pieces having a parallel portion diameter of 9 mm were alsomachined from the steel blanks and tested for tensile strength.

The thus-determined tensile strength and deformation are also summarizedin Table 1. The present inventive steels had tensile strengths in arange of 708 MPa to 992 MPa and deformations of less than 0.40 while theconventional QT (quenched and tempered) steel (No. 1, quench-hardenedfrom 850° C., tempered at 600° C.) and the conventional microalloyedforging steel (No. 2) had deformations of 0.56 to 0.65. Comparativesteel No. 12 had a relatively small deformation. However, a furtherstudy showed that, because of a large carbon content, sample No. 12 hadas small a yield ratio as 0.58 and was inferior to the present inventivesample Nos. 6 and 41 having relatively small yield ratios of 0.64 and0.62 because they had the largest carbon contents among the presentinventive samples. The comparative sample Nos. 19 and 21 contained largeamounts of Al and had a poor machinability which was 20% lower than thatof sample No. 15 in terms of VL1000 (the maximum circumferential speedat which drilling can be conducted for a total drilled length of 1000mm) measured by using a cemented carbide drill.

INDUSTRIAL APPLICABILITY

As described hereinabove, the present inventive steel has a goodstrength and an extremely small deformation upon fracture as a machinestructural steel having a ferrite-pearlitic microstructure forautomobile and industrial machinery use, and moreover, is inexpensive.The present inventive steel is most advantageously applied inferrite-pearlitic steel blanks and parts not requiring a good impactproperty but subject to fracture working.

                                      TABLE 1                                     __________________________________________________________________________                                                              Deform-                                                                   T.S.                                                                              ation               No.                                                                              Note       C  Si Mn P   S   Cr Mo V   Nb TN  Others                                                                              (MPa)                                                                             (mm)                __________________________________________________________________________     1 Comparative QT steel                                                                     0.55                                                                             0.23                                                                             0.72                                                                             0.012                                                                             0.020                                                                             0.02                                                                             -- --  -- 0.0076                                                                              --  746 0.64                 2 Comparative micro-                                                                       0.50                                                                             0.25                                                                             0.80                                                                             0.017                                                                             0.058                                                                             0.28                                                                             -- 0.055                                                                             -- 0.0082                                                                              --  788 0.56                   alloyed forging steel                                                       3 1st invention                                                                            0.55                                                                             0.41                                                                             0.24                                                                             0.024                                                                             0.014                                                                             -- -- 0.153                                                                             -- 0.0047                                                                              --  866 0.31                 4 1st invention                                                                            0.31                                                                             1.86                                                                             0.19                                                                             0.013                                                                             0.054                                                                             -- -- 0.152                                                                             -- 0.0022                                                                              --  708 0.38                 5 1st invention                                                                            0.43                                                                             0.80                                                                             0.22                                                                             0.022                                                                             0.055                                                                             -- -- 0.200                                                                             -- 0.0032                                                                              --  836 0.35                 6 1st invention                                                                            0.58                                                                             0.50                                                                             0.25                                                                             0.023                                                                             0.060                                                                             -- -- 0.151                                                                             -- 0.0028                                                                              --  894 0.39                 7 1st invention                                                                            0.52                                                                             0.11                                                                             0.34                                                                             0.023                                                                             0.085                                                                             -- -- 0.177                                                                             -- 0.0031                                                                              --  887 0.35                 8 1st invention                                                                            0.53                                                                             0.50                                                                             0.35                                                                             0.055                                                                             0.092                                                                             -- -- 0.204                                                                             -- 0.0020                                                                              --  947 0.32                 9 1st invention                                                                            0.45                                                                             1.33                                                                             0.22                                                                             0.094                                                                             0.053                                                                             -- -- 0.204                                                                             -- 0.0025                                                                              --  894 0.32                10 1st invention                                                                            0.31                                                                             0.42                                                                             0.17                                                                             0.020                                                                             0.175                                                                             -- -- 0.385                                                                             -- 0.0035                                                                              --  955 0.30                11 Comparative steel                                                                        0.20                                                                             1.50                                                                             0.28                                                                             0.020                                                                             0.005                                                                             -- -- 0.200                                                                             -- 0.0034                                                                              --  691 0.40                12 Comparative steel                                                                        0.70                                                                             0.22                                                                             0.50                                                                             0.030                                                                             0.060                                                                             -- -- 0.110                                                                             -- 0.0034                                                                              --  979 0.38                13 Comparative steel                                                                        0.49                                                                             1.02                                                                             0.35                                                                             0.022                                                                             0.054                                                                             -- -- 0.170                                                                             -- 0.0090                                                                              --  887 0.49                14 Comparative steel                                                                        0.50                                                                             0.98                                                                             0.36                                                                             0.022                                                                             0.055                                                                             -- -- 0.173                                                                             -- 0.0146                                                                              --  895 0.52                15 2nd invention                                                                            0.45                                                                             1.29                                                                             0.29                                                                             0.088                                                                             0.046                                                                             -- -- 0.197                                                                             -- 0.0021                                                                            Al: 0.026                                                                           891 0.30                16 2nd invention                                                                            0.53                                                                             0.44                                                                             0.37                                                                             0.062                                                                             0.085                                                                             -- -- 0.198                                                                             -- 0.0036                                                                            Al: 0.047                                                                           926 0.32                17 3rd invention                                                                            0.42                                                                             0.50                                                                             0.14                                                                             0.030                                                                             0.110                                                                             -- -- 0.250                                                                             -- 0.0035                                                                            Ti: 0.014                                                                           860 0.36                18 3rd invention                                                                            0.41                                                                             1.01                                                                             0.12                                                                             0.024                                                                             0.108                                                                             -- -- 0.205                                                                             -- 0.0029                                                                            Ti: 0.045                                                                           823 0.38                19 Comparative steel                                                                        0.46                                                                             1.20                                                                             0.30                                                                             0.030                                                                             0.109                                                                             -- -- 0.210                                                                             -- 0.0026                                                                            Al: 0.088                                                                           912 0.44                20 Comparative steel                                                                        0.47                                                                             1.05                                                                             0.22                                                                             0.028                                                                             0.121                                                                             -- -- 0.201                                                                             -- 0.0097                                                                            Ti: 0.080                                                                           889 0.45                21 Comparative steel                                                                        0.43                                                                             0.44                                                                             0.22                                                                             0.024                                                                             0.111                                                                             -- -- 0.200                                                                             -- 0.0122                                                                            Al: 0.067,                                                                          825 0.53                                                                Ti: 0.054                     22 4th invention                                                                            0.45                                                                             0.77                                                                             0.28                                                                             0.023                                                                             0.062                                                                             0.25                                                                             -- 0.182                                                                             -- 0.0022                                                                              --  867 0.33                23 4th invention                                                                            0.47                                                                             0.74                                                                             0.24                                                                             0.025                                                                             0.060                                                                             0.47                                                                             -- 0.182                                                                             -- 0.0024                                                                              --  902 0.34                24 3rd invention                                                                            0.49                                                                             0.73                                                                             0.26                                                                             0.024                                                                             0.062                                                                             -- -- 0.160                                                                             0.07                                                                             0.0022                                                                              --  937 0.31                25 3rd invention                                                                            0.31                                                                             0.20                                                                             0.20                                                                             0.025                                                                             0.059                                                                             -- -- 0.155                                                                             0.18                                                                             0.0029                                                                              --  879 0.32                26 4th invention                                                                            0.49                                                                             0.30                                                                             0.26                                                                             0.022                                                                             0.084                                                                             -- 0.15                                                                             0.175                                                                             -- 0.0034                                                                              --  902 0.32                27 3rd & 4th inventions                                                                     0.40                                                                             0.33                                                                             0.28                                                                             0.022                                                                             0.080                                                                             -- 0.48                                                                             0.170                                                                             -- 0.0029                                                                              --  971 0.33                28 3rd & 4th inventions                                                                     0.39                                                                             0.55                                                                             0.30                                                                             0.025                                                                             0.055                                                                             0.30                                                                             -- 0.162                                                                             0.06                                                                             0.0019                                                                              --  872 0.37                29 3rd invention                                                                            0.39                                                                             0.54                                                                             0.32                                                                             0.024                                                                             0.052                                                                             0.29                                                                             0.20                                                                             0.152                                                                             -- 0.0021                                                                              --  860 0.39                30 3rd & 4th inventions                                                                     0.43                                                                             0.55                                                                             0.30                                                                             0.025                                                                             0.051                                                                             -- 0.27                                                                             0.168                                                                             0.05                                                                             0.0020                                                                              --  992 0.39                31 3rd & 4th inventions                                                                     0.40                                                                             0.02                                                                             0.21                                                                             0.040                                                                             0.052                                                                             0.10                                                                             0.12                                                                             0.151                                                                             0.06                                                                             0.0027                                                                              --  641 0.35                32 2nd & 4th inventions                                                                     0.35                                                                             0.65                                                                             0.27                                                                             0.032                                                                             0.021                                                                             0.20                                                                             -- 0.160                                                                             -- 0.0038                                                                            Al: 0.015                                                                           749 0.38                33 2nd & 3rd & 4th                                                                          0.37                                                                             0.82                                                                             0.25                                                                             0.033                                                                             0.022                                                                             0.22                                                                             0.34                                                                             0.163                                                                             -- 0.0032                                                                            Al: 0.015,                                                                          918 0.37                   inventions                                   Ti: 0.010                     34 3rd & 4th inventions                                                                     0.36                                                                             0.84                                                                             0.25                                                                             0.030                                                                             0.019                                                                             -- 0.20                                                                             0.164                                                                             0.07                                                                             0.0027                                                                            Ti: 0.013                                                                           923 0.31                35 Comparative steel                                                                        0.33                                                                             0.20                                                                             0.24                                                                             0.025                                                                             0.066                                                                             1.02                                                                             -- 0.155                                                                             -- 0.0032                                                                              --  783 0.43                36 Comparative steel                                                                        0.35                                                                             0.11                                                                             0.38                                                                             0.032                                                                             0.060                                                                             -- 0.90                                                                             0.157                                                                             -- 0.0038                                                                              --  1106                                                                              0.47                37 Comparative steel                                                                        0.35                                                                             0.13                                                                             0.35                                                                             0.030                                                                             0.062                                                                             -- -- 0.160                                                                             0.29                                                                             0.0070                                                                              --  1101                                                                              0.47                38 1st invention                                                                            0.55                                                                             0.33                                                                             0.29                                                                             0.032                                                                             0.056                                                                             -- -- 0.202                                                                             -- 0.0034                                                                            Pb: 0.10,                                                                           934 0.37                                                                Ca: 0.001                     39 2nd invention                                                                            0.49                                                                             0.51                                                                             0.20                                                                             0.024                                                                             0.123                                                                             -- -- 0.199                                                                             -- 0.0050                                                                            Al: 0.029,                                                                          861 0.37                                                                Pb: 0.01,                                                                     Ca: 0.0008                    40 3rd invention                                                                            0.45                                                                             0.16                                                                             0.17                                                                             0.054                                                                             0.055                                                                             -- -- 0.150                                                                             -- 0.0033                                                                            Ti: 0.015,                                                                          751 0.37                                                                Bi: 0.05                      41 3rd invention                                                                            0.60                                                                             0.12                                                                             0.12                                                                             0.097                                                                             0.180                                                                             0.20                                                                             -- 0.175                                                                             -- 0.0020                                                                            Te: 0.02                                                                            912 0.36                __________________________________________________________________________     (Note)                                                                        Chemical compositions in wt %. "T.S." means tensile strength". "3rd & 4th     inventions" etc. mean "combination of the third and fourth inventions"        etc.                                                                     

We claim:
 1. A microalloyed forging steel for machine structural use,characterized by consisting of:C : 0.3 to 0.6 wt %, Si: 0.1 to 2.0 wt %,Mn: 0.1 or more and less than 0.4 wt %, p : 0.01 to 0.1 wt %, S : 0.01to 0.2 wt %, V : more than 0.15 wt % and up to 0.4 wt %, and thebalance: Fe and unavoidable impurities, in which the unavoidableimpurities include less than 0.005 wt % N and the steel has aferrite-pearlite microstructure.
 2. A microalloyed forging steel formachine structural use according to claim 1, characterized by furthercontaining:Al: 0.005 to 0.05 wt %.
 3. A microalloyed forging steel formachine structural use according to claim 1 characterized by furthercontaining one or both of:Ti: 0.005 to 0.05 wt %, and Nb: 0.05 to 0.2 wt%.
 4. A microalloyed forging steel for machine structural use accordingto claim 1 characterized by further containing one or both of:Cr: 0.1 to0.5 wt %, and Mo: 0.1 to 0.5 wt %.
 5. A microalloyed forging steel formachine structural use according to claim 1 characterized by furthercomprising at least one of:Pb: 0.01 to 0.4 wt %, Bi: 0.01 to 0.4 wt %,Se: 0.01 to 0.4 wt %, Te: 0.002 to 0.005 wt %, and Ca: 0.0005 to 0.003wt %.